1. Field of the Invention
This invention relates to the injection system of an injection molding machine. More particularly, the present invention relates to an anti-backflow valve positioned at the forward end of a plasticating screw to prevent backflow of plasticated material into the screw flights of plasticated material ahead of the screw upon forward axial movement of the screw to effect injection of the plasticated material into a mold cavity.
2. Description of the Related Art
In plastic molding machines utilizing a plasticating screw to convey polymeric material through a heated barrel so that it undergoes softening, becomes plasticated, and is capable of flowing, the screw is rotated to carry plasticated material along the screw to its forward end. As plasticated material accumulates ahead of the screw, the screw is permitted to move axially rearwardly until a predetermined volume of plasticated material has been accumulated ahead of the screw. At that point rotation of the screw is terminated, and the screw is caused to travel axially forward toward an outlet in the barrel, to thereby force the plasticated material through the outlet and inject it into an adjacent mold cavity, whereupon a part of predetermined configuration can be formed upon cooling of the material in the mold cavity.
During the injection step, while the plasticating screw is moving axially toward the outlet of the barrel, it is necessary that backflow of plasticated material from the space ahead of the screw into the screw flights be prevented, so that the desired predetermined quantity of plasticated material is completely injected into the mold cavity in order to form a complete molded part.
Various types of non-return or anti-backflow valve structures carried on the forward end of a plasticating screw have been devised over the years, and examples of commonly used valve structures are illustrated and described in U.S. Pat. No. 4,377,180, which issued on Mar. 22, 1983, to William H. Biljes, and U.S. Pat. No. 4,477,242, which issued on Oct. 16, 1984, to Martin Eichlseder et al. The non-return valves illustrated and described in those patents each include an annular sealing ring that is slidable a limited distance in the direction of the screw axis. When the screw is rotating and is plasticating material, the annular ring is in a forwardmost position, and plasticated material is permitted to flow from the forward flights of the screw through the center portion of the ring. When axial movement of the screw toward the barrel outlet commences, the annular sealing ring slides rearwardly, relative to the screw, and contacts a cooperating sealing surface on the axially moving screw to close off the flow path between the screw flights and the screw tip, and thereby permit injection into a mold of a desired portion of the volume of plasticated material that has been accumulated ahead of the forward end of the screw. However, both the Biljes and Eichlseder et al. structures, which are similar to a number of other non-return valve constructions, each include a screw tip around which the annular ring is carried, and that tip is firmly secured to the forward end of and rotates with the screw. It has been found that at times when the machine is first started, and plastic material in solidified form is present within the barrel, unless the plasticated material has been softened by heating before rotation of the screw is initiated, which involves some delay, it is possible for the tip of the screw to break because the tip is surrounded by and is tightly gripped by the cool, solid plastic material as the screw is being rotated. Additionally, because the screw tip serves as a forward stop for the annular sealing ring during plastication, and because there is relative rotational movement between the screw tip and the sealing ring, both the screw tip and the sealing ring are required to be made from hard, wear-resistant materials in order to avoid the need for frequent replacement of those parts because of wear.
One form of anti-backflow valve that overcomes the problems of the previous-identified prior art devices is disclosed in U.S. Pat. No. 4,850,851, which issued Jul. 25, 1989, to Alex Dinerman, one of the inventors of the invention disclosed in the present application. The Dinerman '851 patent discloses an anti-backflow valve in which the screw tip is rotatable relative to the screw, to thereby overcome the screw tip breakage problem referred to above. The Dinerman valve also includes an annular slider or sealing ring that is capable of axial movement relative to the screw to provide sealing action and thereby control the flow of material through the valve.
West German Offenlegungschrift No. 2,515,530, in the name of Wilhelm Fischer, and French Patent No. 2,363,429, in the name of Marcel Manceau, each disclose anti-backflow valves that permit relative rotation between the screw and the screw tip. However, those publications do not disclose any arrangement for preventing the flow of plasticated material into the blind bore in the forward end of the screw.
It is an object of the present invention to overcome the problems attendant with the prior art anti-backflow valve constructions and to provide a valve structure that permits the use of less expensive materials.
It is a further object of the present invention to provide an anti-backflow valve in which the screw tip will not break on start-up of screw rotation if the plastic material surrounding the tip during a cold start is not sufficiently softened.
It is a still further object of the present invention to provide an anti-backflow valve that includes fewer parts and that is therefore less expensive to manufacture.
It is another object of the present invention to provide an anti-backflow valve that does not permit the flow of plasticated material into the forward end of the screw that receives the screw tip.
Other objects and advantages of the present invention will become apparent from the ensuing description.